Fundamentals of Adhesion and Composite Technology

Plant cell walls are about five micrometer thick and consist of hundreds of strengthening cellulose nanofibril layers. We want to understand how these cell walls are composed and how they function as composite material.

The mechanical function in plant structures is primarily provided by natural nanocomposite materials, the reinforcing phase consisting of nanoscale cellulose microfibrils. This program involves studies of material function of different plant structures. The goal is improved understanding of cell wall properties in a cellulose nanocomposites perspective.

Furthermore, we study the design and fabrication of nanocomposites based on cellulose microfibrils. The cellulose used for preparation of new materials can either be disintegrated from plant cell walls or bacterial cellulose.

An interesting characteristic of plant cell walls is their high mechanical performance despite high water content. The first attempts towards biomimetic nanocomposites have focused on materials based on a cellulose microfibril network in a viscous polysaccharide matrix. Nanocomposite foams, films and aerogels have also been produced, where cellulose microfibrils are included in the cell wall.

With the assistance of advanced technology, we study the interaction between different cellulose surfaces in nanoscale. Fundamental knowledge about friction and adhesion at a molecular level can for instance result in better performance in papermaking and recycling or improved strength properties of novel biocomposites.

The same forces that exist between molecules, control the chemical and physical processes in and around interfaces of different materials. The adhesion in polysaccharide-based material is of significant importance for the material properties. For example, in papermaking the adhesion between cellulose fibres largely decides the strength of the paper. One of the things that we study within Biomime is xyloglucan, a plant polymer that binds very strongly to cellulose in plant cell walls. Xyloglucan is an interesting molecule since it allows both very low friction and strong adhesion at the same time. In papermaking these two parameters are just what the producers like to have on cellulose fibres – but the parameters are rarely united.

This program focuses on the surface modification of cellulose using a wide range of activities, spanning from pure chemistry and advanced polymer chemistry to biotechnology for surface modification and sophisticated new analytical technology for characterization. The effects of innovative cellulose surface modification are evaluated both in terms of chemical properties as well as surface properties, such as wettability, friction and adhesion.

Environmentally friendly composites

The objective is to be able to pinch ideas from that knowledge to make new biomimetic materials, says Lars Berglund, Prof. at KTH Fiber & Polymer Chemistry.

Traditionally many industrial composite materials are oil based. In biocomposite materials, wood cellulose, or starch from potatoes or corn, is used instead. Cellulose and starch are biodegradable and renewable resources, which have the additional advantage that they don’t increase the net discharge of carbon dioxide.

– Cellulose and starch are biodegradable and renewable resources that don’t increase the net discharge of carbon dioxide, which is an advantage, says Lars Berglund.

In the KTH laboratory the researchers tailor different types of nanocomposites based on cellulose nanofibrils; films, foam and aerogels. The latter is an extremely porous material consisting to 99 percent of air, used for example in filter and separation applications.

The research group uses nanofibrils from bacterium cellulose, produced by the Biomime, but they also extract nanofibrils from wood cellulose. The nanofibril extracting technique has been developed in collaboration with Innventia, and will, according to Prof. Lars Berglund, be industrially commercialized. There are o lot of different applications for cellulose nanofibrils, functional food being one of them.

One goal of the research group is to replace Styrofoam, used in packaging materials, with foam based on starch and cellulose nanofibers. Foam materials based on starch already exist, but they are extremely sensitive to moisture and compared to polystyrene foam, have not as good properties. Hopefully, with increased knowledge about cellulose nanofibrils combined with nanotechnology, it is possible to replace oil-based plastics and improve the starch foam properties.

– The first step is to show that it is doable, that the concept works, which we have succeeded to do. We have made starch foam that contains a very high amount, 40 percent, of cellulose nanofibre, says Lars Berglund.